The present invention relates to electromagnetic interference shields, and more particularly to such shields for fiber-optic modules.
The need for electromagnetic interference (EMI) shielding in fiber-optic modules is well known in the art. Typically, the fiber-optic module resides on the end of a printed circuit board and protrudes through a metallic enclosure that encloses the board and the fiber-optic module. The enclosure""s faceplate comprises openings, through which a fiber-optic connector can couple to the fiber-optic module. The electronic components on the circuit board and the fiber-optic module generate electromagnetic (EM) radiation. Since high powers of EM radiation may pose a health hazard, the amount of radiation that is allowed to leak out of such enclosures is limited by government regulations. Typically, without special care, the gaps between the fiber-optic module and the enclosure opening tend to allow unacceptable amounts of electromagnetic radiation generated in the enclosure to leak through. Thus it is well know in the art that an electromagnetic interference (EMI) shield placed around the fiber-optic module is necessary to close these gaps and prevent high EM radiation levels from leaking from the enclosure. The EMI shield is made out of conductive material and couples the fiber-optic module with the enclosure to prevent the EMI radiation from leaking through the gaps and to electrically ground the fiber-optic module.
FIG. 1 illustrates a conventional EMI shield and module. The conventional EMI shield 100 comprises a sheet of metal folded around the housing 104 of the module. The shield 100 comprises a plurality of springs 102 which would contact the enclosure faceplate 106 at its opening 108. This contact grounds the module 104 and prevents EMI radiation from leaking through the gap between the module 104 and the enclosure faceplate 106, reducing EMI radiation leakage. It is well know that that for proper EM shielding, the gaps between the contact points of the springs 102 and the enclosure faceplate 106 must be small for efficient shielding.
There are several disadvantages of the prior art. The use of springs 102 that have the compression force that pushes outward from the module 100 along the x and the y-axes, i.e., parallel with the enclosure faceplate 106, requires the springs 102 and the enclosure opening 108 to have a tight tolerance. The reason is that the module 104 and the printed circuit board on which the module is mounted must be precisely placed in the enclosure opening 108 to realize adequately uniform pressure on all springs 102 to ensure proper contact between the shield 100 and the enclosure faceplate 106. Furthermore, the shield 100 is costly to manufacture as it requires multiple manufacturing steps. The metal is first cut to form a flat pattern of the shield 100, including a plurality of fingers. The fingers are then bent to form the springs 102. The flat pattern is then folded to fit over the module 104. Additionally, to realize springs with sufficient compression force to make contact with the enclosure 106 necessitates wide springs and ultimately lower density of springs, further limiting the efficiency of the shielding scheme.
Accordingly, there exists a need for an improved EMI shield for fiber-optic modules. The improved EMI shield should have looser tolerance for easier placement of the module and should be less costly to manufacture. The present invention addresses such a need.
An improved EMI shield for a module includes springs in which the compression force pushes the springs in a direction that is approximately perpendicular to the optic axis. The springs allow significantly looser tolerances on the placement of the module on its associated printed circuit board. The shield also includes an opening near its center, the edge of which includes a plurality of teeth. When the shield is placed onto the module, the teeth bend along the negative optic axis direction and prevent the shield from being pulled away from the module. Accidental removal of the shield is thus prevented. The shield splits the enclosure (chassis) ground from the internal signal ground, decreasing the capacitance between them, in turn decreasing another possible source of electromagnetic radiation. In addition, the shield is more cost efficient to manufacture since it only requires a one step assembly process.